1. Field
This patent specification describes a sheet elevation apparatus for use in an image forming apparatus, and more particularly to a sheet elevation apparatus capable of being coupled to an image forming apparatus with a more easy-to-use manner. This patent specification further describes an image forming apparatus using the above-mentioned sheet elevation apparatus.
2. Discussion of the Background Art
Background image forming apparatuses such as copiers, facsimiles, and printers have been provided with a sheet supply box for containing a stock of recording sheets. A typical example of the sheet supply box is called a sheet tray cassette which is detachably mounted to an image forming apparatus. Some sheet tray cassettes include a sheet elevating mechanism for elevating a stock of recording sheets placed on a base plate upwardly in accordance with an amount of recording sheets remaining on the base plate. With this mechanism, an uppermost recording sheet of the stock of recording sheets placed on the base plate is kept at a predetermined position suitable for being picked up for a transportation to an image forming operation.
In responding to an increasing demand for a high-volume reproduction, a recent sheet tray cassette has a relatively large capacity and needs a relatively great driving power for elevation of the base plate. In addition, a so-called front-loading is a current mainstream of the sheet tray cassette, capable of being loaded into an image forming apparatus from a front position, that is, an operator's position. The front-loading sheet tray cassette is typically provided with a coupling to engage an elevator with a drive motor. When the sheet tray cassette is pulled out, the coupling disengages the elevator from the drive motor which is left in the apparatus.
In one example coupling mechanism, a joint which forms a coupling at the drive motor side is movable in an axis direction by a predetermined stroke and has an edge surface having a cross-shaped groove formed in a direction perpendicular to the axis. A counter member forming the coupling together with the joint has a top surface forming a cross-shaped pin. The cross-shaped pin of the counter member is inserted to the cross-shaped groove of the joint so that a power of the drive motor is transmitted.
The above-mentioned joint is pressed toward the counter member in the axis direction with a spring. The cross-shaped pin and the cross-shaped groove may not always conveniently be met with each other for an appropriate engagement during a loading of the sheet tray cassette into the image forming apparatus. When the cross-shaped pin and the cross-shaped groove are not properly met and are not engaged with each other, the counter member pushes the joint in a direction opposing the force of the spring. As the joint is moved in the direction opposing the force of the spring, a relative position of the joint and the counter member is gradually changed. When the relative position of the joint and the counter member is changed to an engagement position, the cross-shaped pin of the counter member is inserted into the cross-shaped groove of the joint. Thereby, an engagement of the coupling is achieved.
The background image forming apparatus is also provided with a pair of rails on which the sheet tray cassette is slid when it is loaded. Loading the sheet tray cassette is not so easy because of weights of recording sheets and the sheet tray cassette itself. Loading the sheet tray cassette also needs a counter power to oppose the force of the spring associated with the joint.
As such, elevating the base plate needs a relatively great power, and the coupling may easily be disengaged if the engagement of the coupling is insufficient. As a result, a the base plate falls down about a fulcrum. To address this problem, a solution may be to make the moving stroke of the joint and the force of the spring both greater. Accordingly, a more greater force for pushing the sheet tray cassette is needed.
In the above case, the sheet tray cassette continues to receive a force in a direction to be pushed out due to a reaction force generated by the spring until an engagement of the coupling is completed. That is, an extra holder may be needed for holding the sheet tray cassette against the reaction force.